In recent years, there have been actively made research and development of not only industrial robots but also consumer robots, which play various roles for people's livelihood. Among such robots, human-like robots (humanoid robot) being able to walk in erect posture are expected to be able to substitute for actions of human beings. In such humanoid robots, many joint parts are provided for imitating actions or motions of a human being, and a variety of motions having a plurality of degrees of freedom in the joint parts are required. For that reason, in each of the joint parts of the humanoid robots, the number of actuators such as servo-motors mounted therein increases corresponding to the degrees of freedom given to each joint part, with the result that it becomes difficult to reduce the size of each joint part due to this, and the structure and arrangement of the actuators are made complicated.
Here, for example, in patent literature 1 (in particular, FIG. 3), there is disclosed a structure for rotationally driving an arm unit of a robot around a roll axis. Specifically, a servo-motor for roll rotation is arranged in the interior of a housing of a shoulder part which is adjacent to a robot body and with which the arm unit is connected, and an output of the servo-motor is connected to the roll axis or shaft through a belt. As a result of this, rotation of an output shaft of the servo-motor will be transmitted as rotation around the roll axis.